Conventionally, a method of separating the light flux of an object image and directing it to a photo-sensing element for the purpose of focus detection has been proposed. For example, Japanese Laid-Open Patent Application Publication No. 63-195630 relates to a camera equipped with a zoom lens, and discloses a technology in which the light flux is split at an intermediate point along the light path of the zoom lens and focus detection is conducted using this split light flux. In addition, Japanese Laid-Open Patent Application Publication No. 2003-140246 discloses an invention relating to a digital single-lens reflex camera that focuses a primary object image formed by an image forming optical system onto a two-dimensional photo-sensing sensor such as a CCD sensor or a CMOS sensor, photoelectrically converts the optical image thus obtained, and obtains image output for that object.
In the foregoing proposals, the light flux of an object image is divided by a splitter or other light flux splitter and separately incident on the image sensor and the focus detection sensor, with detection of the state of focus carried out on the focus detection sensor side and image sensing carried out on the image sensor side.
In the conventional examples described above, the splitter or other such light flux splitter is provided within the light flux of an object image. However, using such a light flux splitter generally diminishes the amount of light that reaches the image sensor. To counteract this effect, the light flux splitter may be retracted during image sensing so that all the light strikes the image sensor, but retracting the light flux splitter with every sensing of an image leads to shutter time lag.